This invention relates to heat treatment of cadmium mercury telluride and, more particularly, relates to heat treatment of homogeneous single crystals of cadmium mercury telluride to improve and impart desired characteristics.
Cadmium mercury telluride, hereinafter referred to as CMT, is a pseudobinary compound with compositions represented by the general formula (Cd.sub.x Hg.sub.1-x).sub.y Te.sub.1-y, wherein x has values between zero and one and y has values between 0.49 and 0.51. Homogeneous single crystals of CMT may be prepared by any one of a number of methods. To correct defects such as vacancies or dislocations in the crystal lattice and to eliminate compositional gradients, and to make p-n type conversions, this CMT single crystal material is usually subjected to a heat treatment or annealing process in the presence of mercury and sometimes in the presence of mercury and one or more of the other CMT constituent elements.
According to U.S. Pat. No. 3,468,363, which issued on Sept. 23, 1969, ingots of CMT are prepared and annealed at 300 degrees C. for one hour under a partial mercury vapor pressure of 150 mm. No information is provided on the Hg source. According to U.S. Pat. No. 3,514,347, which issued on May 26, 1970, CMT is subjected to thermal treatments to obtain n-type material or to achieve p-n type conversion. The heat treatments are carried out at specified temperatures and low pressures of mercury. It is stated that the source of mercury vapor is placed in a zone cooler than the CMT, i.e., a temperature gradient exists.
According to U.S. Pat. No. 3,723,190, which issued on Mar. 27, 1973, CMT is annealed under mercury vapor obtained from a source of excess mercury while a temperature gradient between the CMT and the mercury source is maintained. The mercury pressure exceeds the "intrinsic line" shown. According to U.S. Pat. No. 3,725,135, which issued on Apr. 3, 1973, epitaxial layers of CMT are subjected to an isothermal heat treatment at 300 degrees C. at a mercury pressure of added mercury of 115 mm. This disclosure is silent on the nature and the temperature of the source of mercury and the temperature of the isothermal heat treatment can, therefore, only relate to the temperature of the epitaxial layer of CMT.
According to U.S. Pat. No. 3,954,518, which issued on May 4, 1976, CMT is annealed at a temperature below the solidus temperature while a constituent vapor pressure lower than the equivalent vapor pressure of the constituents of the CMT is maintained. The mercury pressure is kept low by controlling the mercury temperature below the temperature of the CMT. Thus, a temperature gradient is used. According to U.S. Pat. No. 3,963,540, which issued on June 15, 1976, CMT material is prepared and subsequently annealed at a temperature near but below the solidus temperature in the presence of excess constituents, giving an excess of mercury vapor alone or an excess of mercury, cadmium and tellurium vapors combined.
According to U.S. Pat. No. 3,979,232, which issued on Sept. 7, 1976, CMT is isothermally annealed in the presence of an excess mercury vapor and cadmium vapor from a source of excess mercury and a source of excess cadmium, while a tellurium vapor source may also be present. According to U.S. Pat. No. 4,028,145, which issued on June 7, 1977, CMT is annealed isothermally in the presence of excess cadmium vapor. According to U.S. Pat. No. 4,116,725, which issued on Sept. 26, 1978, CMT is annealed in presence of excess mercury and tellurium using a temperature gradient between the CMT and the mercury and tellurium.
Thus, the prior art discloses two basic methods for the heat treatment of CMT in the presence of one or more constituents of CMT. The first method generally comprises heat treatment of CMT using a differential temperature or temperature gradient, whereby the CMT is at one temperature and the source(s) of added constituent(s) is (are) at a second temperature. The vapor pressure(s) of the constituent(s) in this method is (are) usually less than the saturation pressure of the constituent(s) at the temperature of the CMT. The second method generally comprises heat treatment of CMT under isothermal conditions and a constituent pressure equal to the saturation pressure at the isothermal temperature. Both these methods require an excess amount of constituent(s) to be present at all times during the heat treatment; such excess(es) being present as liquid(s) or solid(s) discrete from the CMT being treated.
However, some disclosures in the prior art are insufficient to determine the exact nature of the annealing method used. For example, it is stated in some cases that annealing is carried out with slices at a defined temperature and under a mercury vapor pressure below the mercury saturation pressure, but the temperature of the mercury source is not taught. Thus, in these cases, in order to obey Gibbs phase rule, the excess mercury must be present at a temperature which differs from the specified temperature, i.e., a temperature gradient is used. Conversely, in those cases wherein the annealing is carried out isothermally in the presence of an excess of constituent(s), it follows that the vapor pressure(s) of the constituent(s) is (are) equal to the saturation vapor pressure of the constituent(s) at the annealing temperature. The main disadvantage of the prior art processes, wherein an excess of constituent element(s) and, particularly, an excess of mercury is present during the heat treatment, is the scavenging by the excess mercury of CMT's constituent elements by dissolution of constituents from the CMT slices via the gas phase into or precipitation onto the excess of added constituent element, especially mercury.